Not Applicable
Not Applicable
The present invention is related to the field of optical communication, and more particularly to optical communications receivers.
A typical structure for the front end of an optical communications receiver includes an optical-to-electrical (O/E) converter such as a positive-intrinsic-negative (PIN) diode, a transimpedance amplifier for converting the current signal produced by the O/E converter to a corresponding voltage signal, and a multi-stage limiter amplifier that produces a higher-amplitude binary data signal from the voltage signal generated by the transimpedance amplifier. The limiter amplifier distinguishes between logic xe2x80x9c0xe2x80x9ds and logic xe2x80x9c1xe2x80x9ds in the received data signal by comparing the data signal with a predetermined switching threshold. If the voltage level of the data signal is greater than the switching threshold, the limiter amplifier produces an output voltage at a first level indicative of a logic xe2x80x9c1xe2x80x9d, and if the voltage level of the data signal is less than the switching threshold, the limiter amplifier produces an output voltage at a second level indicative of a logic xe2x80x9c0xe2x80x9d.
It is important that the switching threshold be established such that the data signal generated by the limiter amplifier resembles the original data signal transmitted to the receiver as closely as possible. In many receiver circuits, the value of the switching threshold is a constant determined by the design of the circuit, and the value is chosen in accordance with known or assumed worst-case operational characteristics it has also been known to employ a calibration procedure in which the power of the received optical signal in an operational system is measured, and a pre-established table is consulted to select a corresponding value for the switching threshold. The first approach may result in an overly conservative receiver design, which can lead to inefficiencies in the optical communication system that might otherwise be avoided. The second approach can achieve a better match between the characteristics of a given optical communications signal and the switching threshold employed by the receiver. However, an explicit calibration procedure cannot be used during normal operation of a communications link, and therefore cannot be relied upon to achieve optimal operation as operating conditions vary. Additionally, because the table is generated in advance, its contents reflect certain assumptions that may or may not be met in any given operational environment. Accuracy and efficient operation may suffer when operational conditions do not reflect these assumptions.
In accordance with the present invention, an optical receiver circuit is disclosed that is capable of quickly and accurately adjusting its switching threshold so as to optimize the reception of data. The receiver circuit is adaptive to different operating conditions, and its operation can also be controlled in a manner providing additional advantages not obtained by the above-described prior approaches.
The disclosed receiver circuit includes a differential limiter amplifier receiving on one differential input a binary data signal generated by an optical-to-electrical converter from a received optical communications signal. Low-pass filter circuits are coupled to respective inverting and non-inverting outputs of the limiter amplifier to obtain their average DC values. The outputs of the low-pass filter circuits are provided to respective inputs of a differential amplifier in order to obtain the difference between these average DC values. The output of the differential amplifier is provided to an integrator circuit whose slowly-varying output is provided to the other differential input of the limiter amplifier to establish a switching threshold for distinguishing logic xe2x80x9c0xe2x80x9ds from logic xe2x80x9c1xe2x80x9ds in the received data signal.
By detecting the difference between the average DC values of the differential signals generated by the limiter amplifier, the receiver circuit can establish a switching threshold as a function of the separation between the logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d levels of the received data signal. This threshold is automatically adjusted in response to varying operation conditions such as changes in the received optical signal power. This operation is achieved without the need for extensive calibration procedures or other complicated techniques.
The disclosed receiver circuit can also receive a generally variable set point input that provides for external control of the switching threshold. Set point adjustment circuitry can be employed to further optimize the setting of the switching threshold, for example to minimize the effects of noise appearing in the received optical signal. This setting can be a relatively constant value, or it may be adjusted dynamically during normal operation of the receiver.
The disclosed receiver circuit can also be used with error-detection circuitry and performance monitoring circuitry to characterize the sensitivity of the error rate in the recovered data signal to the value of the switching threshold. A quality factor or xe2x80x9cQxe2x80x9d factor can be derived from this information, and made available to management software for use in characterizing and/or modifying system behavior. In particular, this information can be used to adjust transmit signal power to improve the optical signal-to-noise ratio (OSNR) of the optical signal. When used in a wavelength-division multiplexed (WDM) system, this mechanism allows for independent adjustment of channel powers to achieve a desired balance among the respective OSNRs of the channels.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.